In recent years, various kinds of information apart from speech, such as images and data, have come to be transmitted in radio communications, and particularly in mobile communications. With the demand for still higher-speed transmission expected to continue to grow in the future, there is a need for a radio transmission technology that achieves high transmission efficiency through more efficient use of limited frequency resources in order to perform high-speed transmission.
One radio transmission technology capable of meeting such a need is OFDM (Orthogonal Frequency Division Multiplexing). OFDM is a multicarrier transmission technology that performs parallel transmission of data using a plurality of subcarriers, and is known for such features as high frequency efficiency and reduced inter-symbol interference in a multipath environment, and for its effectiveness in improving transmission efficiency.
Studies have been carried out into performing frequency scheduling transmission when this OFDM is used in a downlink, and data for transmission to a plurality of radio communication mobile station apparatuses (hereinafter referred to simply as mobile stations) is frequency-domain-multiplexed on a plurality of subcarriers.
In frequency scheduling transmission, a radio communication base station apparatus (hereinafter referred to simply as a base station) assigns subcarriers adaptively to mobile stations based on the received quality of each frequency band at each mobile station, enabling maximum multi-user diversity to be obtained. On the other hand, frequency scheduling transmission is normally performed for individual resource blocks in which a number of adjacent subcarriers are collected together into a block, and therefore it is not possible to obtain a very great frequency diversity effect.
In order to perform frequency scheduling transmission, prior to data transmission a base station transmits control information comprising a mobile station ID (user ID), resource block number, data channel modulation scheme and coding ratio (Modulation and Coding Scheme: MCS), and so forth, at the start of each subframe for a data transmission destination of each subframe. This control information is transmitted via an SCCH (Shared Control Channel). There are a number of SCCHs equal to the number of mobile stations to which data is transmitted in the relevant subframe, and the number of mobile stations per subframe is stipulated by the available frequency bandwidth (hereinafter referred to as system bandwidth) in the communication system or the like, for example. That is to say, at the start of each subframe, a number of SCCHs equal to the number of data channels in that subframe are multiplexed at the same time.
Also, with SCCHs, transmission power control is performed on a mobile station by mobile station basis. In this transmission power control, a plurality of SCCHs share power resources within the permissible transmission power (maximum transmission power) of a base station, and an SCCH for a mobile station near a cell boundary is controlled at high transmission power while an SCCH for a mobile station in the central area of a cell is controlled at low transmission power. By means of this transmission power control, limited power resources can be utilized efficiently by being shared in a flexible manner among the SCCHs of individual mobile stations.
Meanwhile, current standardization studies suggest that it is necessary for the MCS of an SCCH to be set to an MCS that meets a 95% coverage target—that is, an MCS such that 95% of all mobile stations within a cell can meet a required received quality. Consequently, an MCS with a rather low MCS level has hitherto been fixed as the MCS of an SCCH. For example, an MCS with QPSK as a modulation scheme and an coding ratio of R=1/8 is set as a fixed MCS that meets a 95% coverage target (see Non-patent Document 1).    Non-patent Document 1: 3GPP RAN WG1 Meeting document, R1-061278